This invention relates generally to methods for operating an engine. More particularly, this invention relates to methods for introduction of a plurality of fuels into an internal combustion engine for operating such an engine.
Internal combustion engines have been widely used in many fields, such as vehicles due to high power-to-weight ratios together with high fuel energy density. In recent years, with increasing attention of reducing pollution of harmful engine emissions to environment, internal combustion engines, such as compression ignition engines (also known as “diesel” engines) can be modified to burn a plurality of fuels, such as both natural gas and diesel fuels, which is referred to as dual fuel operation.
During the dual fuel operation, the natural gas is premixed with an intake air, and then the mixture is introduced into engine cylinders during intake strokes thereof. Subsequently, compression strokes of the engine cylinders start and proceed to increase pressure and temperature of the mixture. At the end of the compression strokes, a small quantity of the diesel fuel is injected into the engine cylinders to ignite the mixture of the intake air and the natural gas by the auto-ignition of the diesel fuel so as to trigger combustion in the engine cylinders.
As a result, the substitution of the natural gas to the diesel fuel reduces the emissions of the pollutants, such as nitrogen oxides (NOx), and particulate matter (PM). However, when the operation of such engines is switched from a steady state to an increased load state, such as a transient acceleration state, the quantity of the natural gas is increased significantly to provide desirable power, which results in reduction of the air fuel ratio (AFR) and may cause the auto-ignition of the premixed natural gas accordingly. The auto-ignition of the premixed natural gas may cause detonation or knocking in the engine cylinders, thereby damaging the engine.
There have been attempts to avoid the detonation or knocking in the engine cylinders. For example, to enable the dual fuel operation, the compression ratio of the engine cylinders may be reduced. This may result in reduction of the cycle efficiency of the engines and increased challenges during cold start. Alternatively, during the dual fuel operation, the substitution of the natural gas to the diesel fuel may be reduced, which increases the harmful engine emissions and the fuel cost due to the increased quantity of the diesel fuel.
Therefore, there is a need for new and improved methods for operating an internal combustion engine in an increased load state thereof while having a high ratio of a gaseous fuel, such as natural gas to a liquid fuel, such as diesel.